Referring to Prior Art FIGS. 1A, 1B, suppliers of photomasks 1 commonly apply a thin pellicle 6 over the mask 8 to protect it, after the final fabrication process has been completed. A pellicle 6 may also protect the bottom side 5 of the photomask 1. Such pellicles 6 may stay on the photomask 1 during its operational use to avoid dust particles that may otherwise contaminate the top layer of the mask 8. Spacing between the thin pellicle 6 and the top layer and/or the bottom 5 of the mask 8 as well as sealing is provided by a surrounding frame(s) 4. The spacing provides on one hand for sufficient clearance such that the thin pellicle 6 does not come into contact with the top layer irrespective eventual sagging of the pellicle 6. This may be particularly an issue with thin pellicles 6 commonly made of organic plastic material with thickness of about less than 10 micron, which are commonly employed with photomasks 1 operated during semiconductor fabrication in conjunction with light sources in the visual spectrum. Such thin pellicles 6 are highly homogeneous and introduce little and type consistent optical distortion. On the other hand, the spacing keeps dust articles that eventually deposit on top of the pellicle sufficiently far above the focal plane at the top layer such that beam distorting effects of eventual dust particles are kept to a minimum.
At the time of this invention and to the inventors knowledge, another form of pellicles 6 are emerging in conjunction with semiconductor fabrication techniques that employ ever shorter wave lengths in the ultraviolet light spectrum. Since organic plastic materials have significantly limited translucency for ultraviolet light and degrade relatively fast in such light, inorganic materials may increasingly need to be used for the pellicle 6. Such materials are commonly very brittle and are made with thickness of 100 microns up to about to 200 microns. With such thickness, the pellicle 6 may introduce optical distortions such as spherical aberrations to the light propagating through. Moreover and due to fabrication cost limitations, the inorganic pellicle 6 may be of limited optical quality with fabrication inconsistencies across its surface area that may introduce localized distortion characteristics. Another potential source of pellicle related optical distortions of the measurement is a well known localized thinning and thinning transitions of organic thin pellicles 6 exposed to radiation near 190 nm and below.
The photomask 1 needs to be inspected preferably through the pellicle 6, once fabrication is finished and prior to the operational use of it. Common photomask inspection devices have the optics very close to the photomask making unimpeded photomask inspection through the pellicle 6 difficult if not impossible to accomplish. In particular, thin film pellicles 6 may bulge in case of a thermal rise of the sealed volume. Hence, there exists a need for a photomask inspection device and method that provides for optical inspection of the photomask 1 with sufficient clearance to the pellicle. The present invention addresses this need.
Because of the emerging thick pellicles 6 for UV photomasks 1 that are visually hard to discern from thin pellicles 6, there exists a need for automatically identifying thin and thick pellicles 6 as well as type of thin and thick pellicles 6. The present invention addresses also this need. Further more and in cases of identified thick pellicles 6, there exists a need for identifying pellicle 6 in homogeneities and automated mapping of eventual localized optical distortion properties of the thick pellicle 6. The present invention addresses also this need.
During photomask inspection, a report of a number of well known properties of the photomask 1 itself is generated to document the quality of the fabricated photomask 1. In cases of thin and thick pellicles 6 used in the near and full UV spectrum, there exists also a need for documenting mapped local optical distortion properties of the pellicle 6, which may be also attributed to well known localized pellicle 6 thinning in cases of organic thin pellicle materials. The present invention addresses also this need.